Terms of service (commonly abbreviated as ToS or TOS and also known as Terms of Use, Terms & Conditions) are rules which one must agree to abide by in order to use a service. Terms of Service can also be referred to as Terms of Use or sometimes merely a Disclaimer, especially regarding the use of websites.

Certain websites are noted for having carefully designed terms of service, particularly eBay and PayPal, which need to maintain a high level of community trust because of transactions involving money. Terms of service can cover a range of issues, including acceptable user behavior online, a company's marketing policies, and copyright notices. Some organizations, such as Yahoo!, can change their terms of service without notice to the user base.

Most Internet service providers have terms of service—notably AOL—which many have perceived to be very strict. Many users do not read the terms of service.^{[citation needed]}

As video games move online, many of them are also adopting terms of service, separate from the software license agreement required to utilize the client software. These Terms of Service generally specify behaviors that are disallowed within the video game's virtual world, generally to prevent a player from disrupting another player's game experience. For example, many online games disallow the use of racist speech through communication channels provided by the game. Many MMORPGs also disallow the trading of in-game items with "real world" money.